In recent years, it is common to periodically observe and measure the biomarkers of a variety of diseases in order to prevent disease, suppress the progression of disease, or monitor health. For instance, diabetic patients have to measure the blood sugar concentration (blood sugar level) three or four times a day. Hyperlipidemia patients have to regularly measure blood cholesterol levels and neutral fat levels, although less frequently than with the blood sugar concentration. Therefore, there is a demand for the development of a handy medical measurement device which can be used by normal people who are not health-care workers.
Blood components are usually measured for blood sugar concentration or blood cholesterol levels after the blood is drawn through a person's skin. However, because blood drawing is painful and involves troublesome operations, such as the sterilization of the blood drawing area and the instruments, and for other reasons too, there is a need for a non-invasive method for measuring membranes (biomembranes), such as the human skin and the retina of a living body. The development of such a non-invasive measurement method is a common issue in the entire field of medicine.
An effective non-invasive method for obtaining information of the components inside a biomembrane is a measurement of the spectral characteristics using a near-infrared light which adequately penetrates the skin. However, when measuring a biomembrane, the object to be measured inevitably moves due to breathing and the beating of the heart. In addition, blood sugar level sensors or other devices for use in daily life are required to be small, portable, and inexpensive.
As a technology for measuring spectral characteristics, methods using the spectroscopic technology known as a wavelength-dispersive spectroscopy or a Fourier spectroscopy have been proposed (refer to Non-Patent Document 1).
The wavelength-dispersive spectroscopy uses the principle that: when a light passing through, or reflected on the surface of, a sample to be measured (which is referred to as an “object light” hereinafter) is delivered to a diffraction grating, its diffraction angle differs depending on the wavelength of the object light.
The Fourier spectroscopy is a spectroscopic measurement technology using a phase-shift interferometry by means of a Michelson-type two-beam interference optical system. Object light is divided into two beams by a beam splitter such as a half mirror, and each beam is reflected by a mirror and delivered again to the half mirror, where both beams merge with each other and the interference phenomenon is observed. The mirror which reflects one (reference light) of the two divided beams is called a reference mirror. In the Fourier spectrometry, the reference mirror is moved with high precision, i.e. with a resolution smaller than the wavelength of light so as to change the intensity of the interfering light, whereby a so-called interferogram is detected. The interferogram is mathematically Fourier-converted to obtain the spectral characteristics.
The object light rays emitted from the surface of the sample to be measured go in a variety of directions due to scattering, refraction, reflection, and other factors. If the light components of various directions are delivered to a diffraction grating and a reference mirror, the spectral accuracy will be deteriorated. Considering this factor, in any spectroscopies, in order to enhance the spatial coherency of the object lights, a pinhole or a slit with a minute opening is used so that only the light component of a specific direction of the object light is delivered to the diffraction grating and the reference mirror. A pinhole having a diameter of tens of microns is used for the dispersive spectroscopy, while a slit having a width of a few millimeters is used for the Fourier spectrometry, although their sizes depend on the required spectral performances.
When a pinhole or a slit is used, most of the object light does not pass through the pinhole or the slit and will not be used for measurement. That is, light efficiency is low. The scattered light and other light generated inside a biomembrane are weak. Conventional spectroscopic techniques are not suitable for a weak light measurement, and therefore it is difficult to observe light scattered in an arbitrary position inside a biomembrane and evaluate its spectral characteristics.
In view of this, the inventor of the present invention has proposed a method for obtaining the interferogram of an object to be measured by using the phenomenon of interference between the beams from the object (object beams), which are generated at each of the bright point that optically constitute the object to be measured (refer to Patent Document 1).
In this method, the light rays generated at each bright point are introduced to a fixed mirror unit and a movable mirror unit of a phase-changeable filter by way of an objective lens. The movable mirror unit is driven by a piezo element or other element. The object beams reflected by these two mirror units form an interference image on the imaging plane. The intensity of the interference image changes as the movable mirror unit moves, thereby forming a so-called interferogram. By Fourier-transforming the interferogram, the spectral characteristics (spectrum) of the transmitted light or diffused/scattered light can be obtained.
In the method according to Patent Document 1, all the rays which have passed through the objective lens can be used for a measurement. That is, the light efficiency is high and therefore the method is suitable for a weak light measurement. In addition, this method does not require a beam splitter, which is an inevitable component for a two-beam interferometer as typified by a Michelson interferometer. That is, the use of reflective lenses for the objective lens and the imaging lens enables the provision of a spectral device employing a reflection optical system. In this case, since the adverse effects of the light dispersion due to transmissive optical elements is avoided, the spectral characteristics across a wide band can be obtained.